1. Field of the Invention
This invention relates to processing of 60 Hz 3232 pulldown format video signals.
2. Description of the Prior Art
24 Hz (24 frames/s) film is commonly converted to 60 Hz (60 fields/s) video using a technique known as 3232 pulldown. This technique is illustrated in FIG. 1 of the accompanying drawings which shows the relationship between the 24 Hz film frames, numbered 1, 2, 3 etc in the figure, and the 60 Hz video fields derived therefrom. Five video fields are produced from each successive pair of film frames as indicated by the arrows in the figure. Firstly, an odd video field 10 is generated by scanning every other line in film frame 1. An even field 1E is then generated by scanning the alternate lines in film frame 1. The odd field 10 is then repeated. An even field 2E and an odd field 20 are then derived from film frame 2 to complete the five field sequence. This process is theft repeated for film frames 3 and 4, but in this case the sequence starts with an even field 3E to maintain the alternating odd and even field sequence, and this even field 3E is repeated. Film frames 5 and 6 are converted in the same manner as frames 1 and 2, and so on. As indicated in the figure, the resulting 60 Hz video signal consists of successive groups of 3 and 2 video fields, the fields in each group being derived from the same frame of film, and one field in each group of three being repeated.
Such 24 Hz film-originated 3232 pulldown material can be converted for display in accordance with a 50 Hz, rather than a 60 Hz, standard. In order to do this, it is necessary for the field sequence of the 60 Hz video to be determined. However, the 3232 field sequence may have been disrupted by editing as shown in FIG. 2 of the accompanying drawings. This figure illustrates how a video edit between a scene A and a scene B has disrupted the 3232 field sequence of the 60 Hz video, the video fields corresponding to the first, second, third etc film frames in each scene being labelled 1, 2, 3 etc in the figure. To allow for the presence of disruptions in the sequence due to editing, therefore, the field sequence must be monitored during processing of the 60 Hz video.
A system for converting 60 Hz 3232 pulldown format video for display at 50 Hz is described in international patent application number WO91/06182. According to this system, the repeated fields in the input 60 Hz signal are identified by calculating the difference between alternate fields in the input signal, ie successive fields of the same polarity. A repeated field is indicated by a zero difference. One field of each pair of repeated fields is eliminated, and the remaining fields are reordered as necessary to provide the required alternating field polarity. The output is a 48 Hz video signal, successive pairs of fields corresponding to respective frames of the original 24 Hz film. The 48 Hz signal can be recorded and replayed at 50 Hz, though there will of course be a 4% increase in the motion rate in the replayed image. More importantly, the output is frame-based since each odd/even field pair is derived from the same frame of film. The effect of this is to produce "film-like" motion portrayal in the resulting video image, and since interlaced fields derived from the same film frame are displayed one after the other, double imaging may be perceived when displayed.
Good motion portrayal in standards converted video signals can be achieved by a technique known as motion compensated temporal interpolation. This technique is described in detail in U.S. Pat. No. 5,005,077, issued Apr. 2, 1991, and corresponding to UK patent application number, and the disclosure in said U.S. Patent is incorporated herein by this reference for example. Briefly, however, the technique may involve: producing a series of progressive scan format frames from the fields of an input video signal, one progressive scan frame being produced for each input field; detecting motion in areas of the image between pairs of progressive scan frames by comparing blocks of pixels in each progressive scan frame with blocks of pixels in the following frame and deriving motion vectors representing the motion of the content of respective blocks between the two frames; and producing output fields/frames at the required temporal positions for the output standard, the pixels in each output frame being derived from pixels in a pair of the progressive scan frame as in dependence upon the motion vectors and the temporal offset between the output field/frame and the two progressive scan frames from which it is formed. In effect, interpolation is performed along the direction of movement of parts of the image between progressive scan frames to produce output fields/frames representing the image as if originally acquired at the output field/frame rate. This process provides high quality motion portrayal in the resulting video image.